How to get the type of protobuf message from the field descriptor?

[Iranjeet Singh]

I am trying to get the type of a data member inside a protobuf message and call the appropriate message from it.

message ControlMessage{}
message AdminMessage{}
message ExecMessage{}

message IncomingMessage
{
  optional ControlMessage cmsg = 1 ;
  optional AdminMessage amsg   = 2 ;
  optional ExecMessage  emsg   = 3 ;
}

void onMsg(const ControlMessage&){}
void onMsg(const AdminMessage&){}
void onMsg(const ExecMessage&){}

int main()
{
  IncomingMessage msg ;
  msg.mutable_cmsg() ;
  msg.mutable_emsg() ;

  auto metadata = msg.GetMetaData() ;
  auto reflection = metadata.reflection ;
  auto descriptor = metadata.descriptor ;

  std::vector<const FieldDescriptor*> all_fields ;

  reflection->ListFields( msg, &all_fields ) ;

  for(auto msgItr : all_fields)
  {
     // Somehow call the onMsg() overloads
     // for all the fields set inside the msg
     // msgItr is the FieldDescriptor
  }
}

[Adam Cozzette]

You can access the messages reflectively and static_cast them to the concrete type. Just make sure you're casting to the correct type since otherwise it will be undefined behavior. Something like this should work:

switch ((*msgItr)->number()) {

  case IncomingMessage::kCmsgFieldNumber:

    onMsg(static_cast<ControlMessage>(reflection->GetMessage(msg, *msgItr)));

  case IncomingMessage::kAmsgFieldNumber:

    ...

}

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